In a modern petroleum refinery several process units generate hydrocarbon byproducts of low commercial value. Side streams comprising lower molecular weight paraffins are generally low in value and often burned as a source of fuel.
Efforts to upgrade hydrocarbon streams containing C.sub.2 -C.sub.10 paraffins have included contacting the paraffins under high severity dehydrocyclization conditions with a crystalline shape selective medium pore siliceous acid catalyst in a fluidized bed reaction zone to obtain aromatic hydrocarbons such as benzene, toluene and isomeric xylenes (BTX). Such an operation provides a convenient one-step route for producing an aromatics-rich gasoline with a relatively high octane number.
U.S. Pat. No. 3,827,968 (Givens et al) discloses a two-stage operation wherein a mixed feed containing paraffins and olefins is upgraded in the absence of added hydrogen to a highly aromatic gasoline product. In a first stage oligomerization reaction, olefins are upgraded to higher molecular weight liquid hydrocarbons under relatively mild conditions with a catalyst having the structure of ZSM-5. In a second stage, said liquid hydrocarbons are converted to an aromatic product. A gas phase highly paraffinic stream is withdrawn as by-product from the first stage.